…5859) As part of ongoing work on improving instance lifecycle management (see #5749) , we intend to remove the `InstanceRuntimeState` tracking from `sled-agent`, and make Nexus the sole owner of `instance` records in CRDB, with a new `instance-update` saga taking over the responsibility of managing the instance's state transitions. In order to properly manage the instance state machine, Nexus will need information about the status of active migrations that are currently only available to sled-agents. For example, if an instance is migrating, and a sled agent reports that the source VMM is `Destroyed`, Nexus doesn't presently have the capability to determine whether the source VMM was destroyed because the migration completed successfully, or that the source shut down prior to starting the migration, resulting in a failure. In order for Nexus to correctly manage state updates during live migration, we introduce a new `migration` table to the schema for tracking the state of ongoing migrations. The `instance-migrate` saga creates a `migration` record when beginning a migration. The Nexus and sled-agent APIs are extended to include migration state updates from sled-agents to Nexus. In *this* branch, the `migration` table is (basically) write-only; Nexus doesn't really read from it, and just stuffs updates into it. In the future, however, this will be used by the `instance-update` saga. It occurred to me that, in addition to using the migration table for instance updates, it might also be useful to add an OMDB command to look up the status of a migration using this table. However, I decided that made more sense as a follow-up change, as I'd like to get back to integrating this into #5749. Fixes #2948

want mechanism to track & report on live migration statuses #2948

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